1READV(2)                   Linux Programmer's Manual                  READV(2)


6       readv,  writev, preadv, pwritev, preadv2, pwritev2 - read or write data
7       into multiple buffers


10       #include <sys/uio.h>
12       ssize_t readv(int fd, const struct iovec *iov, int iovcnt);
14       ssize_t writev(int fd, const struct iovec *iov, int iovcnt);
16       ssize_t preadv(int fd, const struct iovec *iov, int iovcnt,
17                      off_t offset);
19       ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt,
20                       off_t offset);
22       ssize_t preadv2(int fd, const struct iovec *iov, int iovcnt,
23                       off_t offset, int flags);
25       ssize_t pwritev2(int fd, const struct iovec *iov, int iovcnt,
26                        off_t offset, int flags);
28   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
30       preadv(), pwritev():
31           Since glibc 2.19:
32               _DEFAULT_SOURCE
33           Glibc 2.19 and earlier:
34               _BSD_SOURCE


37       The readv() system call reads iovcnt buffers from the  file  associated
38       with the file descriptor fd into the buffers described by iov ("scatter
39       input").
41       The writev() system call writes iovcnt buffers of data described by iov
42       to the file associated with the file descriptor fd ("gather output").
44       The  pointer  iov  points  to  an array of iovec structures, defined in
45       <sys/uio.h> as:
47           struct iovec {
48               void  *iov_base;    /* Starting address */
49               size_t iov_len;     /* Number of bytes to transfer */
50           };
52       The readv() system call works just like read(2)  except  that  multiple
53       buffers are filled.
55       The  writev() system call works just like write(2) except that multiple
56       buffers are written out.
58       Buffers are processed in array order.  This  means  that  readv()  com‐
59       pletely fills iov[0] before proceeding to iov[1], and so on.  (If there
60       is insufficient data, then not all buffers pointed to  by  iov  may  be
61       filled.)   Similarly, writev() writes out the entire contents of iov[0]
62       before proceeding to iov[1], and so on.
64       The data transfers performed by readv() and writev()  are  atomic:  the
65       data  written  by  writev()  is  written  as a single block that is not
66       intermingled with output  from  writes  in  other  processes  (but  see
67       pipe(7) for an exception); analogously, readv() is guaranteed to read a
68       contiguous block of data from the file, regardless of  read  operations
69       performed  in  other  threads  or  processes that have file descriptors
70       referring to the same open file description (see open(2)).
72   preadv() and pwritev()
73       The preadv() system call combines  the  functionality  of  readv()  and
74       pread(2).   It  performs  the  same  task as readv(), but adds a fourth
75       argument, offset, which specifies the file offset at  which  the  input
76       operation is to be performed.
78       The  pwritev()  system  call combines the functionality of writev() and
79       pwrite(2).  It performs the same task as writev(), but  adds  a  fourth
80       argument,  offset,  which specifies the file offset at which the output
81       operation is to be performed.
83       The file offset is  not  changed  by  these  system  calls.   The  file
84       referred to by fd must be capable of seeking.
86   preadv2() and pwritev2()
87       These system calls are similar to preadv() and pwritev() calls, but add
88       a fifth argument, flags, which modifies  the  behavior  on  a  per-call
89       basis.
91       Unlike  preadv()  and pwritev(), if the offset argument is -1, then the
92       current file offset is used and updated.
94       The flags argument contains a bitwise OR of zero or more of the follow‐
95       ing flags:
97       RWF_DSYNC (since Linux 4.7)
98              Provide  a  per-write  equivalent  of  the O_DSYNC open(2) flag.
99              This flag is meaningful only  for  pwritev2(),  and  its  effect
100              applies only to the data range written by the system call.
102       RWF_HIPRI (since Linux 4.6)
103              High priority read/write.  Allows block-based filesystems to use
104              polling of the device, which provides lower latency, but may use
105              additional  resources.   (Currently, this feature is usable only
106              on a file descriptor opened using the O_DIRECT flag.)
108       RWF_SYNC (since Linux 4.7)
109              Provide a per-write equivalent of the O_SYNC open(2) flag.  This
110              flag  is  meaningful only for pwritev2(), and its effect applies
111              only to the data range written by the system call.
113       RWF_NOWAIT (since Linux 4.14)
114              Do not wait for data which is  not  immediately  available.   If
115              this  flag  is  specified, the preadv2() system call will return
116              instantly if it would have to read data from the backing storage
117              or wait for a lock.  If some data was successfully read, it will
118              return the number of bytes read.  If no bytes were read, it will
119              return  -1  and  set  errno  to EAGAIN.  Currently, this flag is
120              meaningful only for preadv2().


123       On success, readv(), preadv() and preadv2() return the number of  bytes
124       read;  writev(),  pwritev()  and  pwritev2() return the number of bytes
125       written.
127       Note that it is not an error for a successful call  to  transfer  fewer
128       bytes than requested (see read(2) and write(2)).
130       On error, -1 is returned, and errno is set appropriately.


133       The  errors  are  as  given  for  read(2)  and  write(2).  Furthermore,
134       preadv(), preadv2(), pwritev(), and pwritev2() can also  fail  for  the
135       same  reasons  as  lseek(2).   Additionally,  the  following errors are
136       defined:
138       EINVAL The sum of the iov_len values overflows an ssize_t value.
140       EINVAL The vector count, iovcnt, is less than zero or greater than  the
141              permitted maximum.
143       EINVAL An unknown flag is specified in flags.


146       preadv()  and pwritev() first appeared in Linux 2.6.30; library support
147       was added in glibc 2.10.
149       preadv2() and pwritev2() first appeared in Linux 4.6.  Library  support
150       was added in glibc 2.26.


153       readv(),  writev():  POSIX.1-2001,  POSIX.1-2008,  4.4BSD (these system
154       calls first appeared in 4.2BSD).
156       preadv(), pwritev(): nonstandard, but present also on the modern BSDs.
158       preadv2(), pwritev2(): nonstandard Linux extension.


161       POSIX.1 allows an implementation to place a  limit  on  the  number  of
162       items  that  can be passed in iov.  An implementation can advertise its
163       limit by defining IOV_MAX in <limits.h> or at run time via  the  return
164       value from sysconf(_SC_IOV_MAX).  On modern Linux systems, the limit is
165       1024.  Back in Linux 2.0 days, this limit was 16.
167   C library/kernel differences
168       The raw preadv() and pwritev() system calls have call  signatures  that
169       differ  slightly  from  that of the corresponding GNU C library wrapper
170       functions shown in  the  SYNOPSIS.   The  final  argument,  offset,  is
171       unpacked  by  the  wrapper  functions  into two arguments in the system
172       calls:
174           unsigned long pos_l, unsigned long pos
176       These arguments contain, respectively, the low order and high order  32
177       bits of offset.
179   Historical C library/kernel differences
180       To  deal  with  the  fact  that IOV_MAX was so low on early versions of
181       Linux, the glibc wrapper functions for readv() and  writev()  did  some
182       extra  work  if  they  detected  that the underlying kernel system call
183       failed because this limit was exceeded.  In the case  of  readv(),  the
184       wrapper  function  allocated a temporary buffer large enough for all of
185       the items specified by iov, passed that buffer in a  call  to  read(2),
186       copied  data from the buffer to the locations specified by the iov_base
187       fields of the elements of iov, and then freed the buffer.  The  wrapper
188       function  for  writev()  performed the analogous task using a temporary
189       buffer and a call to write(2).
191       The need for this extra effort in the glibc wrapper functions went away
192       with  Linux  2.2  and  later.  However, glibc continued to provide this
193       behavior until version 2.10.  Starting  with  glibc  version  2.9,  the
194       wrapper  functions  provide  this  behavior only if the library detects
195       that the system is running a Linux kernel older than version 2.6.18 (an
196       arbitrarily  selected  kernel  version).   And  since glibc 2.20 (which
197       requires a minimum Linux kernel version of 2.6.32), the  glibc  wrapper
198       functions always just directly invoke the system calls.


201       The following code sample demonstrates the use of writev():
203           char *str0 = "hello ";
204           char *str1 = "world\n";
205           struct iovec iov[2];
206           ssize_t nwritten;
208           iov[0].iov_base = str0;
209           iov[0].iov_len = strlen(str0);
210           iov[1].iov_base = str1;
211           iov[1].iov_len = strlen(str1);
213           nwritten = writev(STDOUT_FILENO, iov, 2);


216       pread(2), read(2), write(2)


219       This  page  is  part of release 4.15 of the Linux man-pages project.  A
220       description of the project, information about reporting bugs,  and  the
221       latest     version     of     this    page,    can    be    found    at
222       https://www.kernel.org/doc/man-pages/.
226Linux                             2017-09-15                          READV(2)